Sterile neutrinos , lepton asymmetries , primordial elements : how much of each ?

We investigate quantitatively the extent to which having a primordial leptonic asymmetry (nν 6= nν̄) relaxes the bounds on light sterile neutrinos imposed by BBN and LSS. We adopt a few assumptions that allow us to solve the neutrino evolution equations over a broad range of mixing parameters and asymmetries. For the general cases of sterile mixing with the electron or muon neutrino, we identify the regions that can be reopened. For the particular case of a LSND-like sterile neutrino, soon to be rejected or confirmed by MiniBooNE, we find that an asymmetry of the order of 10−4 is needed to lift the conflicts with cosmology.